The present invention relates to a light-emitting device, particularly but not exclusively, an electro luminescent device for use as a backlight to an LCD display, and displays incorporating such devices. The electroluminescence for the electroluminescent device may be suitably provided by means of an organic light-emissive material (see for example International Publication WO90/13148).
By way of background, FIG. 1 shows the typical cross-sectional structure of an organic light-emissive device. The device is fabricated on a substrate (1) coated with a transparent first electrode (2) such as indium-tin-oxide. The coated substrate is overcoated with at least one layer of a thin film of an electroluminescent organic material (3) and a final layer forming a second electrode (4) which is typically of metal. By using a transparent substrate (e.g. of glass or plastics material), light generated in the film (3) is able to leave the device by passing through the first electrode (2).
The performance of electroluminescent devices has advanced rapidly over the past few years. Due to their high efficiencies, the devices show potential for a wide range of display applications, from simple backlights to graphic displays, such as television screens, computer monitors and palm-top devices which may consist of several million pixels. In particular, organic light-emissive material may be used in the fabrication of backlights for transmissive or transflective liquid crystal displays. In a liquid crystal display there is typically a planar liquid crystal cell which has active regions where the optical properties of the liquid crystal material can be altered by the application of an electric field to vary the transmission of light through the active regions. In a transmissive liquid crystal display there is a light source behind the liquid crystal cell; and light from the source shines to an observer through those regions permitted to transmit light. In a transflective liquid display, the light source is supplemented by a reflective mirror, also behind the liquid crystal cell, which can return incident light towards the observer.
Organic light-emissive materials have been devised to emit light over a range of colours, and a full set of red- green- and blue-emitting polymers are now available. Emission colour of electroluminescent materials in devices has been controlled in one of several ways. Firstly, it is known to use a semiconductive conjugated co-polymer comprising at least two chemically different monomer units which have different semiconductor band gaps when existing in their individual homo-polymer forms. The relative proportions of the monomer units may be varied to alter the semiconductor band gap so as to control the optical properties of the resultant co-polymer (see WO92/03490). Another approach, particularly suitable for producing a single white light-emitting layer, involves depositing on a substrate a mixture of blue and red-type organic electroluminescent materials using a flash vacuum deposition process. (See JP 0921989).
The present applicant has appreciated the desirability of white light-emitting devices, both for use as point sources as well as backlights for LCD displays of mobile phones and the like. The present applicant has devised a novel device which is able to operate at a lower voltage than white light-emitting, mixed organic electroluminescent material known in the prior art.
In accordance with a first aspect of the present invention, there is provided a light-emitting device comprising a first electroluminescent element for emitting light of a first colour when energised and a second electro luminescent element for emitting light of a second colour when energised, characterised in that dimensions of the first and second elements and any spacing therebetween are selected to create an impression of a single light source when the elements are simultaneously energised and viewed as intended.
The present applicant believes that such a light-emitting device may provide a convenient way of achieving a particular lighting effect, especially if the desired colour of the light is difficult to achieve with a single electroluminescent material. This is because in perceiving a single light source, it is believed the viewer viewing the device in the intended manner will also perceive a single colour of light, determined in part by the sum of the first and second colours. If the first and second colours are different, the single colour of light perceived will also be different.
The first and second elements may be energised by a common bias. In other words, the elements may share the same anode and cathode. Such a construction would be relatively simple to manufacture, and is to be contrasted with known devices where different light emitting regions are independently energised.
In one embodiment, at least one of the electro luminescent elements comprises an organic light-emissive material. The material may be a polymer, perhaps a conjugated polymer. The organic light-emissive material may be deposited on a substrate, perhaps by a process of ink-jet deposition. Ink-jet deposition enables efficient, fine and accurate definition of the at least one electroluminescent element. Each element may have a generally circular profile over the surface of the substrate. Ink-jet deposition could readily deposit such xe2x80x9cdotsxe2x80x9d in a compact and reproducable array (65,000 dots per cm2).
The first electroluminescent element may be one of a plurality of such elements for emitting light of the first colour when energised, the plurality of elements being arranged in a first spaced-apart array. The second electroluminescent element may be one of a plurality of such elements for emitting light of the second colour when energised, the plurality of second light-emitting elements being arranged in a second spaced-apart array. The first and second spaced-apart arrays may overlap, with each element of the first spaced-apart array adjacent to elements of the second spaced-apart array. The first and second spaced-apart arrays may share energising electrodes.
The dimensions of the discrete elements and any spacings between adjacent elements may be less than the limit of resolution of the naked eye, say less than 1.0xc3x9710xe2x88x924 meters when the object is placed at the near point. The maximum dimension of the elements on the surface of the substrate may be less than 5.0xc3x9710xe2x88x926 meters; and the maximum spacing between adjacent elements in the plane of the substrate may be less than 5.0xc3x9710xe2x88x926 meters. In one form, the spacing between adjacent elements may be negligible or even non-existent (i.e. adjacent elements contact a common insulator). Such fine dimensions and spacings may be particularly useful in small (hand held) electronic articles incorporating the light-emitting device, where visual acuity and achieving maximum fill factor (minimising dark areas) are important considerations. However, with larger articles, which are intended to be viewed from distances greater than one arm""s length, the size of and spacings between elements become less critical. This is because the minimum feature size resolvable to the naked eye is dependent on viewing distance (i.e distance between object and observer). The angular resolving power of the naked eye remains constant, so as the distance increases, so does the size of the minimum feature which is resolvable.
The first and second colours of the first and second organic light-emissive materials may be selected from the group consisting of red, green and blue. By selecting one red emitter and one green emitter, it may be possible to give the impression of a substantially white light-emitting device. The device may further comprise a third electroluminescent element for emitting light of a third colour, the dimensions of the third element and any spacing between the third and adjacent elements being selected to create the impression of a single light source where all the elements are energised and viewed as intended. The first, second and third organic light-emissive materials may be selected to produce red, green and blue light emissions respectively.
The hue or colour temperature of the single light perceived by an observer may be varied by modifying the relative proportions (i.e. areal density) of the different elements. Accordingly, the different arrays may cover different proportions of the substrate, either by varying sizes or numbers of elements. For example, the proportions of the substrate covered by the red, green and blue light-emitting material may be present respectively in the ratio 100:1:3.7(?)
The light emitting elements may be mounted on a substrate and confined within an area of 2.5xc3x97109 m2 (a square of sides 50 xcexcm). Such a light emitting device would be on the same size scale as a light emitting diode, and may thus be regarded as a point source emitter, at least to the naked eye.
There is also provided an electronic device comprising an LCD display and a backlight comprising a light-emitting device in accordance with the first aspect of the invention.
In accordance with a second aspect of the present invention, there is provided a method of manufacturing a light-emitting device, comprising providing a first electroluminescent element for emitting light of a first colour when energised; providing a second element for emitting light of a second colour when energised; characterised by: selecting dimensions of the elements and any spacing therebetween to create an overall impression of a single light source when simultaneously energised and viewed as intended.
The method may further comprise energising the elements with a common bias. This may be achieved by coupling the elements to a common anode and a common cathode.
The method may further comprise providing a plurality of electroluminescent elements for emitting light of the first colour when energised, the plurality of first-colour light-emitting elements being arranged in a spaced-apart array with the first electroluminescent element. The method may also comprise providing a plurality of electroluminescent elements for emitting light of the second colour when energised, the plurality of second-colour-light-emitting elements being arranged in a spaced-apart array with the second electroluminescent element. The spaced-apart arrays of the first and second elements may share energising electrodes. In one embodiment, the method comprises disposing the first electroluminescent element adjacent second-colour-light-emitting elements.
The first or second organic light-emissive materials may be deposited by a process of ink-jet printing.
In another aspect of the invention, there is provided a light-emitting device comprising a first element for emitting light of a first colour, a second element for emitting light of a second colour, and an electrode common to both elements for actuating light emission, wherein the size and any spacings between the first and second elements are selected to create the impression of a single light source when the elements are actuated simultaneously and viewed as intended.
The first and second elements may comprise semiconducting or electroluminescent materials. The light-emitting device may further comprise an additional electrode common to both elements for actuating light emission.